Today, we will discuss the bones of the upper extremity. What I'd like to emphasize as I go, and will refer to it periodically, is that the upper extremity as they often say has one job and one job only, and that's to position the hand to grasp. That will be a theme that we'll be following as we proceed. Our objectives here are to name the major bones of the upper limb, to name the bones at the major joints, and to describe the location of the carpal tunnel because of its importance in movements of the hand. We begin with this view of the entire skeleton and remind ourselves we're talking about the skeleton as we pass off from the shoulder to the hand. The shoulder girdle refers to the clavicle and the scapula, and it's the part of the shoulder that articulates with the axial skeleton, that allows the shoulder to connect with the rest of the skeletal system. That system involves a series of joints. From the midline here we see the sternum, we see the sternoclavicular joint, which connects the sternum and the clavicle, as its name implies, and then distally we see the AC joint or acromioclavicular joint, which connects the clavicle to the acromion, which is a part of the scapula. Then, of course, we see the humerus attaching to the scapula. The shoulder joint itself then consists of the humeral head, which articulates, sits in a very shallow socket called the glenoid cavity. We see adjacent to it the coracoid process, which is an attachment point for muscles that we'll see later, and we see the large flat scapular body. Posteriorly, we see some more detailed anatomy of the scapula. We see the scapular spine, which extends from medial to lateral, and it's large distal enlargement is the acromion. Then, superior or above this spinous process, we see the supraspinous fossa. This is where the supraspinatus muscle, which we'll talk about later attaches. Then, inferior to the scapular spine, we see the infraspinatous fossa. This diagram emphasizes something interesting about the shoulder joint. We see the glenoid fossa, which I said is a shallow socket, and we see the humeral head. We see how much larger that humeral head is than the glenoid fossa. This has a tremendous advantage for us because it allows a great deal of movement of the shoulder joint as we're all aware, our ability to move our arm around in very elaborate ways, but it also makes it unstable. Shoulder dislocations can often occur in violent impacts. This mismatch between humeral head and glenoid fossa is an important feature of the joint to keep in mind. We look at the humerus now. We follow the humerus distally to the elbow, we see the shaft of the humerus. Then, distally we can see that the humerus expands into the elbow joint. The elbow joint here we can see is composed of the radius, and the ulna, and the humerus. The humerus has two major articulation locations there at the elbow. One of them is the capitulum, where the humerus and the radius articulate, and the other is the trochlea, where the ulna and the humerus articulate. This view shows how the ulna wraps around the trochlea of the humerus. We see this prominent process, the, if you will, the bump on the back of our elbow, the olecranon process, which serves as an attachment for muscles that we'll learn about later, but also adds tremendous stability to the elbow joint. Now, if we look distally, we see the radius and ulna passing to the wrist. We can see that the ulna, while being prominent at the elbow, is much smaller at the wrist. By contrast, the radius, which is rather small at the elbow, is very prominent at the wrist. We see the wrist joined here is between the radius and ulna and a series of bones called the carpal bones. The carpus is the wrist. There are numerous bones; there are eight of them we could talk about with the wrist, but I'll concentrate on the ones that are most important. As we can see from this image, the ulna does not really articulate or rather meet with the carpal bones directly. It connects by way of a cartilaginous disc that sits in the space between the ulna and the carpal bones. The main wrist joint is between the radius and two bones, the scaphoid bone and the lunate bone. These are the bones that transmit the force if we fall on our hand, for example. The scaphoid is often damaged in wrist fractures. The other distal bone that I mentioned because of its importance in movements that we'll learn about shortly, is the trapezium bone. Trapezium bone here articulates with the first metacarpal and it's relationship there is responsible for our ability to move our thumb in so many interesting and complicated ways. Looking distally from the carpals, we go to metacarpals. There are five of them, one for each digit. Then distally we see the phalanges. There's a proximal middle and distal phalanx for the second, third, fourth, and fifth digits. For the thumb there's just simply a proximal and distal phalanx. An important structure here is a structure called the transverse carpal ligament, sometimes called the flexor retinaculum. It's a tough band of connective tissue that passes from the medial to lateral carpal bones. We care about it because it's through this carpal tunnel that the tendons that control the fingers, as well as a median nerve, pass out to the hand. As usual, we'll have a low-pressure quiz here just to see how you're doing. The first question here is; the shoulder joint is between: the humerus and scapula, clavicle and humerus, or clavicle and scapula? Take a minute. What do you think? If you said between the humerus and the scapula, that's good. That's the shoulder joint and we talked about the mismatch in size between the humeral head and the glenoid fossa that facilitated movement to a great degree but also render the joint site be unstable. The elbow is stabilized mostly by the joint between: the radius and ulna, the radius and the humerus, and the humerus and ulna? If you said humerus and ulna, that's good. You recall from our image that showed the ulna wrapping around the humerus, the olecranon process and the relationship between the ulna and the trochlea of the humerus makes that part of the joint more stable. Then transfer of weight to the forearm from the wrist involves mostly: the lunate and the ulna, the scaphoid and the lunate, the scaphoid and the radius? If you said scaphoid and radius, that's good. You recall that the ulna is relatively small at the wrist and does not articulate with it directly but through a little wedge of cartilage. It's the radius that's prominent at the wrist and its main articulation is with the scaphoid. We could also add the lunate to that as well, but it's the radius and the scaphoid that are the principal articulation points at the wrist. The transverse carpal ligament runs between; the radius and ulna, medial and lateral carpal bones, or the metacarpal bones? If you said medial and lateral carpal bones, that's good. Our next discussion will involve movements allowed by these joints. Now that we know the major bones involved, we can start thinking about how they work to make the limb and the hand to do their marvelous jobs.
